The paucity of data on the genetic epidemiology of breast cancer for racial/ethnic groups other than those of European ancestry hinders the development of innovative interventions to reduce health disparities. Women in the African Diaspora experience a disproportionate burden of pre-menopausal breast cancer in comparison to all other races for reasons that remain unknown and understudied. This higher proportion of early-onset breast cancer might suggest a stronger genetic component in these populations. Genome-wide association studies (GWAS) have revealed several genetic loci that confer risk of breast cancer. Because all GWAS started the discovery stage in women of European ancestry and replicated mainly in women of European ancestry, we propose a novel approach for a GWAS in indigenous African women to identify alleles associated with breast cancer risk which will then be replicated in other populations. This innovative design builds on our current understanding of the etiologic heterogeneity in breast cancer and the distribution of breast cancer molecular subtypes which differ between women of African ancestry and women of European ancestry. The major objective of the proposed studies is to get to the root causes of breast cancer by identifying breast cancer risk alleles in a pooled sample of women of African ancestry and to replicate our findings in other populations. To achieve this objective, we propose the following specific aims: 1) Genotype 1,796 breast cancer cases and 1,988 controls of African ancestry using the Illumina Human1M BeadChip platform. These include 944 cases and 665 controls form Ibadan, Nigeria, 171 cases and 378 controls from Barbados, and 681 cases and 945 controls from Chicago, Detroit, Philadelphia and Baltimore; 2) Conduct both standard and novel genetic analyses of the data to map genes associated with breast cancer susceptibility, 3) Verify genotyping and carry out fine-mapping studies in genes or regions showing association with breast cancer risk and related clinical phenotypes and 4) Replicate in other African American and non-African American populations. By pooling unique resources from studies throughout the African Diaspora, this study has the potential to identify risk alleles in several genes that contribute to increased breast cancer risk and may have implications for early detection, prognosis and treatment of breast cancer in ALL women. This should ultimately lead to improved outcomes for those who suffer a disproportionate burden of early-onset breast cancer. PUBLIC HEALTH RELEVANCE: Of all the racial/ethnic groups in the United States, African Americans have the highest mortality rate of breast cancer diagnosed in women under 35 years of age, and in Africa, breast cancer is a uniformly fatal affliction of young women, in part, due to poor access to care and ignorance of the disease. This project focuses on the understudied global problem of breast cancer in the African Diaspora and attempts to translate recent advances in genomics research to benefit women who are at risk of developing hormone receptor negative breast cancer, an aggressive form of breast cancer that often affects younger women. Better understanding of the genetic risks of breast cancer for women in the African Diaspora should lead to better clinical risk assessment and the development of more effective strategies for prevention, early detection and treatment of breast cancer for ALL women.